Systems and methods for thermal actuation of microfluidic devices

ABSTRACT

A microfluidic processing device includes a substrate defining a microfluidic network. The substrate is in thermal communication with a plurality of N independently controllable components and a plurality of input output contacts for connecting the substrate to an external controller. Each component has at least two terminals. Each terminal is in electrical communication with at least one contact. The number of contacts required to independently control the N components is substantially less than the total number of terminals. Upon actuation, the components typically heat a portion of the microfluidic network and/or sense a temperature thereof.

RELATED APPLICATIONS

The present application is a continuation-in-part of (a) U.S.application Ser. No. 10/489,404, which is a national stage applicationof international application no. PCT/US02/29012, filed Sep. 12, 2002,U.S. application nos. No. 09/949,763, filed Sep. 12, 2001, now U.S. Pat.No. 6,852,287, and (c) U.S. application Ser. No. 09/819,105, filed Mar.28, 2001, now U.S. Pat. No. 7,010,391. The present application alsoclaims the benefit of U.S. provisional application Nos. 60/491,264,filed Jul. 31, 2003, 60/491,539, filed Aug. 1, 2003, 60/491,269, filedJul. 31, 2003, 60/551,785, filed Mar. 11, 2004, and 60/553,553, filedMar. 17, 2004. All of the foregoing applications are incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates to microfluidic devices, and moreparticularly to systems and methods for operating microfluidic devices.

BACKGROUND

Microfluidic devices are typically configured to manipulate minuteamounts of materials, such as to determine the presence and/or amount ofa target compound within a sample. The devices manipulate materialswithin a microfluidic network, which generally includes elements such asvalves, gates, pumps, reaction chambers, mixing chambers, enrichmentmodules, filtration modules, and detection modules. These elements canbe thermally actuated under computer control.

SUMMARY

One aspect of the invention relates to a microfluidic device forprocessing microfluidic samples. The processing device can includethermally actuated elements, such as one or more of a valve, a pump, ora reaction chamber, configured to manipulate materials, e.g.,microfluidic samples and/or reagents, within the device. The device, ora system configured to operate the device, can also include a pluralityN of independently controllable components, each component having atleast two terminals, at least one of the components configured toactuate the at least one element of the device. For example, a componentmay be configured to actuate a valve and a component may be configuredto actuate a pump of the device. In general the components are heatsources or temperature sensors. In some embodiments, at least some ofthe components are both heat sources and temperature sensors.

The device typically includes a plurality of input/output contacts forelectrically connecting the components to a controller, wherein thenumber of contacts required to independently control the N components issubstantially less than the total number of terminals.

In some embodiments, each contact is electrically connected to at leastone terminal of each of at least two of the N components, and whereinthe terminals of the N components are connected to a unique combinationof the contacts, so that the external controller can control eachcomponent independently of other components.

The device can include a plurality of current flow directional elements,e.g., diodes. Each current flow directional element can be configured toallow current to flow in essentially only one direction through at leastone of the components. An electrical pathway between each contact and atleast one terminal of each component can include a current flowdirectional element. Each component can include a corresponding currentflow directional element.

In some embodiments, at least one of the components includes a pluralityof active regions each active region disposed in thermal contact with arespective thermally actuated element of the microfluidic device. Uponthe passage of current through the at least one component, each of theactive regions generates an amount of heat sufficient to actuate therespective element of the microfluidic device. The active regions arespaced apart by regions that do not generate sufficient heat to actuatea thermally actuated element of the microfluidic device.

In some embodiments, at least one of the components has a temperaturesensitive resistance. The system can include a processor configured toactuate first and second actuation states of the component. In the firstactuation state of the component, the component generates an amount ofheat sufficient to actuate an element of the microfluidic network. In asecond actuation state, a temperature dependent electricalcharacteristic of the component is determined. The temperature dependentelectrical characteristic is indicative of a temperature of thecomponent and, typically, of a portion of the corresponding element ofthe microfluidic element or material therein. The first and secondactuation states of the component can be repeated.

In some embodiments, a method for fabricating a microfluidic processingdevice includes providing a substrate having a plurality of componentseach having at least two terminals and providing a plurality ofinput/output contacts for connecting the substrate to an externalcontroller. A plurality of leads are provided for connecting thecontacts to the terminals. The number of contacts required toindependently control the N components is substantially less than thetotal number of terminals, and wherein the controller can therebycontrol each the component independently of each other component.

The method can include providing a plurality of current flow directionalelements configured to allow current to flow in essentially only onedirection through each of at least some of the N components. The currentflow directional elements are typically diodes.

In some embodiments, a microfluidic system includes a substrateincluding a microfluidic network including at least one of each of athermally actuated valve, a thermally actuated pump, and a thermallyactuated reaction chamber. The system also includes a plurality ofcomponents and a plurality of electrical contacts. Each component is inthermal communication with a respective one of the valve, pump, andreaction chamber. Each contact is in electrical communication with atleast two different components. Each component is in electricalcommunication with at least a pair of contacts. No component of at leasta subset of the components is in electrical communication with the samepair of contacts.

Another aspect of the invention relates to a system for operating amicrofluidic device. The system typically includes a microfluidic devicecomprising a channel configured to receive a fluidic sample, anelectrical pathway comprising a resistive element disposed in thermalcommunication with the channel, the resistive element having atemperature-dependent resistance, an electrical energy source inelectrical communication with the electrical pathway, and an electricalmeasurement device configured to obtain data indicative of an electricalcharacteristic of the resistive element.

The system includes a computer-readable medium comprising: code toprovide a first actuation state of the electrical energy source, whereina first electrical current flows through the resistive element, code toprovide a second actuation state of the electrical energy source. Asecond, lower electrical current flows through the resistive elementduring the second actuation state. The computer-readable medium alsoincludes code to receive data indicative of the electricalcharacteristic of the resistive element from the electrical measurementdevice.

In some embodiments, the computer-readable medium comprises code todetermine a temperature of the resistive element based on the dataindicative of the electrical characteristic of the resistive elementreceived from the electrical measurement device. The data indicative ofthe electrical characteristic of the resistive element can be indicativeof a temperature-dependent resistance of the resistive element. The codecan be configured such that the data indicative of thetemperature-dependent resistance of the resistive element is obtainedwhile the electrical energy source is in the second actuation state.

The data indicative of the electrical characteristic of the resistiveelement may be indicative of an electrical potential required to cause apredetermined current to flow through the resistive element while theelectrical energy source is in the second actuation state.

The computer-readable medium may include code to determine a temperatureof the resistive element based on the data indicative of the electricalcharacteristic of the resistive element received from the electricalmeasurement device. The data may be indicative of the electricalcharacteristic of the resistive element when the electrical energysource is in the second actuation state. Also included is code tocompare the temperature of the resistive element with a predeterminedtemperature value and code to repeat the first and second actuationstates of the electrical energy source if the temperature is less thanthe predetermined temperature value.

The computer-readable medium may include code to compare, based upon thereceived data indicative of the electrical characteristic: the second,lower current and a predetermined current, and code to increase anelectrical potential across the resistive element during the secondactuation state if the second, lower current is less than thepredetermined current, code to decrease an electrical potential acrossthe resistive element during the second actuation state if the second,lower current exceeds the predetermined current, and code to receiveelectrical potential data indicative of the electrical potential acrossthe resistive element during the second actuation state if the second,lower current is within a predetermined range of the predeterminedcurrent.

The computer-readable medium can include code to determine thetemperature of the resistive element based on the electrical potentialacross the resistive element when the second, lower current is withinthe predetermined range of the predetermined current.

The computer-readable medium can include code to provide the firstactuation state of the electrical energy source if the temperature ofthe resistive element is less than a predetermined temperature. Thecomputer-readable medium can include code to repeatedly determine thetemperature of the resistive element based on the electrical potentialacross the resistive element when the second, lower current is withinthe predetermined range of the predetermined current and provide thefirst actuation state of the electrical energy source if the temperatureof the resistive element is less than the predetermined temperature.

In some embodiments, the resistive element has a thermal dissipationconstant (DC) and, during the first actuation state, the code can beconfigured to control the resistive element to dissipate a power k,wherein the ratio k/DC≧40° C., ≧55° C., or ≧65° C. The ratio may bek/DC<300° C., <250° C., <200° C., <175° C., or <150° C.

Another aspect of the invention relates to a method for monitoring atemperature of material present within a channel of a microfluidicdevice. The method can include providing a microfluidic device includinga channel and an electrical pathway comprising a resistive element inthermal communication with the channel. A liquid sample is introducedinto the channel. A first electrical current is caused to flow throughthe electrical pathway by applying a first electrical potential acrossthe resistive element. A second, predefined and lower, electricalcurrent is caused to flow through the electrical pathway by applying asecond electrical potential across the resistive element. A temperatureof the fluidic material is determined based upon the second electricalpotential required to cause the second electrical current to flowthrough the electrical circuit.

The resistive element can be a platinum-comprising conductor having atemperature-dependent resistance.

In some embodiments, a method for operating a microfluidic system tomonitor a temperature of material present within a channel of amicrofluidic device includes providing a microfluidic analysis systemincluding a microfluidic device. The microfluidic device includes amicrofluidic network comprising at least one channel. The systemincludes an electrical pathway comprising a junction between a firstmaterial and a second, different material, at least a portion of one ofthe first and second materials are in thermal communication with thechannel. Application of an electrical current across the junctionincreases a temperature of the at least a portion of one of the firstand second materials. The junction comprises at least onetemperature-dependent electrical characteristic. The system alsoincludes a source of electrical current.

A liquid sample is introduced into the channel. A first electricalcurrent is applied across the junction. The first electrical current isgenerated by the source of electrical current and is sufficient to heatthe at least a portion of one of the first and second materials to atleast 30° C. Data indicative of the at least one temperature-dependentelectrical characteristic of the junction are obtained. A temperature ofthe liquid sample is determined based upon the data.

The step of obtaining data indicative of the at least onetemperature-dependent electrical characteristic can be performed afterperforming the step of applying a first electrical current. Prior to thestep of obtaining data, the method can include reducing the firstelectrical current to an amount insufficient to heat the at least oneportion of one of the first and second materials to at least 30° C.

In some embodiments a method for monitoring a temperature of materialpresent within a channel of a microfluidic device of a microfluidicsystem includes introducing a liquid sample to the channel, applying afirst electrical potential to an electrical pathway in thermalcommunication with the channel, applying a second, lower, electricalpotential to the electrical pathway, and determining a temperature ofthe fluidic material based upon a current that flows through theelectrical pathway when the second potential is applied.

Another aspect of the invention relates to a method for monitoring atemperature of a thermally actuated valve. The method includes providinga microfluidic system including a microfluidic device. The deviceincludes a channel having an upstream portion and a downstream portionand a valve comprising a closed state and an open state. Upon changing atemperature of at least a first portion of the valve, the valvetransitions from one of the closed or open states to the other state.The system also includes an electrical pathway comprising a resistiveelement in thermal communication with the first portion of the valve. Afirst electrical potential is applied to the circuit. A second, lower,potential is applied to the electrical pathway. A temperature of thefirst portion of the valve is determined based upon a current that flowsthrough the electrical circuit when the second potential is applied.

Another aspect of the invention relates to a method of calibrating aresistive heating element of a microfluidic device. The method caninclude manufacturing a first microfluidic device. The first deviceincludes a microfluidic network including a channel configured toreceive a liquid sample therein. The channel is located in thermalcontact with a resistive element of an electrical pathway. A liquidsample is introduced to the channel. The liquid sample includes at leastone component exhibiting a temperature dependent physio-chemicalproperty at a known temperature. An amount of electrical currentrequired to heat the liquid sample to a temperature sufficient toobserve the physio-chemical property of the fluidic sample isdetermined.

A second microfabricated device can be manufactured. The second deviceincludes a channel configured to receive a fluidic sample therein and isconfigured to be operated in thermal communication with a resistiveelement of an electrical pathway. The amount of electrical currentrequired to heat a liquid sample present in the channel of the seconddevice to a predetermined temperature is determined based on the amountof current required to heat the liquid sample in the first device.

Typically, the devices include an injection-molded substrate. Thetemperature is typically between 60° C. and 90° C. The physio-chemicalproperty can be the enzyme-based amplification of a polynucleotide. Thephysio-chemical property may be a phase transition of a temperatureresponse material, e.g., wax.

A computer-readable medium can be provided with current to operate anelectrical energy source to cause an amount of current to flow throughthe resistive element of the second microfluidic device, the amount ofcurrent determined by methods described herein.

Another aspect of the invention relates to a method for calibrating aheat source of a microfluidic system. The method includes manufacturinga first microfluidic device defining a microfluidic network including achannel configured to receive a fluidic sample therein. The device isoperated using a microfluidic system including an electrical pathwaycomprising an electrical element in thermal communication with thechannel. A liquid sample is introduced to the channel. The liquid sampleincludes at least one component exhibiting a temperature dependentphysio-chemical property at a known temperature. An amount of electricalenergy required to heat the fluidic sample to a temperature sufficientto observe the physio-chemical property of the fluidic sample isdetermined.

The method can also include manufacturing a second microfluidic devicedefining a microfluidic network including a channel configured toreceive a liquid sample therein; Instructing a user to operate themicrofluidic device with the channel in thermal contact with a heatsource; and Instructing the user to introduce a fluidic sample to thechannel and providing a computer-readable medium including codeconfigured to actuate the heat source to heat the liquid sample in thechannel. The code is configured to actuate an electrical energy sourceto provide an amount of electrical energy determined on the basis of theamount of electrical energy required to heat the fluidic sample presentin the first microfluidic device to a temperature sufficient to observethe physico-chemical property of the fluidic sample.

Another aspect of the invention relates to a microfluidic systemincluding a microfluidic device defining a microfluidic networkincluding at least one thermopneumatic pressure source. The system alsoincludes at least two thermo pneumatically actuated components ingaseous communication with the thermopneumatic pressure source, whereinpressure within the thermopneumatic pressure source simultaneouslyactuates each of two thermopneumatically actuated components.

In some embodiments, each of the thermopneumatically actuated componentsincludes a respective mass of thermally responsive substance (TRS). Themasses of TRS are spaced apart from one another. The pressure from thepressure source simultaneously moves each of the masses of TRS. Thecomponents may be valves or gates configured to obstruct or allowpassage of sample and/or reagents along a channel. Upon actuation, oneor more of the masses of TRS may pass along the channel in a directionof flow of the sample and/or reagents.

In some embodiments, neither of the at least two thermopneumaticallyactuated components can be actuated independently of the other of the atleast two thermopneumatically actuated components.

Another aspect of the invention relates to a microfluidic system. Thesystem includes a microfluidic device defining a microfluidic networkincluding first and second thermally actuated components. The systemalso includes first and second resistive heat sources in series with andspaced apart by a first more conductive region. The first thermallyactuated component is in thermal communication with the first heatsource and the second thermally actuated component is in thermalcommunication with the second heat source.

Upon the passage of a current through the first heat source, the firstmore conductive region, and the second heat source, the first heatsource generates an amount of heat sufficient to actuate the firstthermally actuated component but insufficient to actuate the secondthermally actuated component and the second heat source generates anamount of heat sufficient to actuate the second thermally actuatedcomponent but insufficient to actuate the first thermally actuatedcomponent. The first more conductive region generates an amount of heatinsufficient to actuate either of the first and second thermallyactuated components.

Other features and advantages of the invention are apparent from thefollowing description, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary microfluidic device;

FIG. 2 is a microfluidic control system having a discrete dropletmicrofluidic processing device, an external controller, and a generalpurpose computer;

FIG. 3 illustrates the discrete droplet microfluidic processing deviceof FIG. 2;

FIG. 4 illustrates the external controller of FIG. 2;

FIG. 5A is a micro-valve actuator in an open state;

FIG. 5B is the micro-valve actuator of FIG. 5A in a closed state;

FIG. 6A is a heat source having a separate resistive temperature sensor;

FIG. 6B is a top view of a microfluidic reaction chamber with a combinedheat source temperature sensor in thermal communication therewith;

FIG. 6C is a side view of the microfluidic reaction chamber and combinedheat source temperature sensor of FIG. 6B;

FIG. 6D is a power dissipation versus time plot of the combined heatsource temperature sensor of FIG. 6B;

FIGS. 7A-B illustrate a micro-valve actuator having a reduced number ofI/O contacts, with the valve being in the open state in FIG. 7A and inthe closed state in FIG. 7B;

FIGS. 8A-B illustrate a technique for sharing conductive leads thatsupply current to resistive heat sources in thermal communication with amicrofluidic device;

FIGS. 9A-B illustrate a technique for sharing conductive leads forcombined heat source temperature sensors, which can be resistivetemperature detectors (“RTDs”);

FIGS. 10A-B illustrate a technique for sharing conductive leads forcombined heat source temperature sensors;

FIG. 11 is a portion of a microfluidic system having a microfluidicdevice defining a microfluidic network in thermal communication withheat sources of a substrate;

FIG. 12A is a side view of a substrate having a combined heat sourcetemperature sensor and a conductive via for providing current to thecombined heat source temperature sensor; and

FIG. 12B is a top view of the substrate of FIG. 12A.

DETAILED DESCRIPTION

Microfluidic devices generally include a substrate that defines one ormore microfluidic networks, each including one or more channels, processmodules, and actuators. Materials, e.g., samples and reagents, aremanipulated within the microfluidic network(s), generally to determinethe presence or absence of some target.

Modules and actuators of typical networks are thermally actuated. Forexample, a process module can include a reaction chamber or lysingchamber that is heated by a heat source. An actuator may include achamber that is heated to generate a pressure or a vacuum to movematerial within the network.

Aspects of the present invention relate to thermal actuation ofcomponents of microfluidic networks. Before undertaking a detaileddiscussion, however, exemplary microfluidic devices, systems andtypically analyzed samples are introduced.

Microfluidic Systems, Devices and Samples

Referring to FIG. 1, an exemplary microfluidic network 110 of amicrofluidic device has a sample input module 150 and reagent inputmodule 152 to allow sample and reagent materials, respectively, to beinput to network 110. Generally, one or both of input modules 150, 152are configured to allow automatic material input using a computercontrolled laboratory robot. Network. 110 may also include output portsconfigured to allow withdrawal or output of processed sample from or bymicrofluidic network 110.

Typical samples include particle-containing fluidic samples. The fluidcomponent of the particle-containing fluidic sample may include a gasand/or, a liquid, e.g., a buffer, water, organic solvents, saliva,urine, serum, blood, or combination thereof. In any case, the fluidtypically entrains the particles such that the particles tend to movewith the fluid.

The particles of the particle-containing fluidic sample generallyinclude cells, such as bacterial cells or cells of an animal, such as ahuman. The particles may include intracellular material released fromsuch cells. For example, the microfluidic systems may detect (uponoptional amplification) polynucleotides, e.g., DNA, released from cells.In some embodiments, the microfluidic system processes DNA released frombacteria cells to determine the presence, absence, and/or abundance ofthe bacteria, e.g., bacteria associated with Group B streptococcal (GBS)disease. Other particles that may be analyzed include tissue, viruses,spores, fungi, and other microorganisms and material released fromwithin such particles.

Within a microfluidic network, sample and reagent materials generallytravel from upstream locations to downstream locations. For example,sample material generally travels downstream from an input port to otherlocations within the microfluidic network. In some cases, however, thedirection of flow may be reversed.

Locations of network 110 downstream from the input module typicallyinclude process modules: enrichment module 156, lysing module 158, DNAclean-up module 160, mixing module 166 and detection module 162 forprocessing the sample and reagent materials. Within these processmodules, a sample is subjected to various physical and chemical processsteps. For example, enrichment module 156 receives a particle-containingfluid and prepares a fluid sample having a relatively higherconcentration of particles. Lysing module 158 releases material fromparticles of an enriched sample, e.g., the module can releaseintracellular material from cells. Lysing can be accomplished using, forexample, thermal, ultrasonic, mechanical, or electrical techniques.Exemplary lysing and enrichment modules are discussed in U.S.provisional application No. 60/491,269, filed Jul. 31, 2003,International application no. PCT/US2004/025181 filed concurrentlyherewith and titled Processing Particle-Containing Sample, and U.S.patent application Ser. No. 10/014,519, filed Dec. 14, 2001, whichapplications are incorporated herein by reference.

DNA clean-up module 160 readies polynucleotides, e.g., DNA, releasedfrom the particles for detection. For example, DNA clean-up module 160can be configured to prepare a DNA sample for amplification bypolymerase chain reaction. Sample DNA processed by clean-up module 160moves downstream within network 110. An exemplary DNA clean-up module isdiscussed in U.S. provisional application No. 60/567,174, filed May 3,2004, which application is incorporated herein by reference.

Mixing module 166 mixes DNA received from DNA clean-up module 160 withreagents from reagent input module 152. Typical reagents include PCRprimers, reagents, and controls. Exemplary reagents are used in theamplification and detection of bacteria, e.g., GBS bacteria. Suchreagents are disclosed in U.S. patent application Ser. No. 10/102,513,filed Mar. 20, 2002, which application is incorporated herein. Reagentmaterials may be loaded during use and/or stored within the microfluidicdevice during manufacturing. Certain reagent materials can belyophilized to extend their storage life. Liquid reagents can be storedwithin a chamber, e.g., a metalized pouch, for mixing with driedreagents.

Detection module 162 receives DNA released from sample particles andreagents and detects minute quantities of DNA therein. In general,detection module 162 is configured to amplify the DNA such as by PCR.Detection is typically spectroscopic, as by fluorescence. In someembodiments, the presence and/or abundance of DNA is detectedelectrochemically.

Detection module 162 typically includes more than oneamplification/detection chamber. One chamber generally receives anddetects (with optional amplification) DNA released from sampleparticles. Another chamber typically receives and detects (with optionalamplification) control DNA, which may be used to indicate whethernetwork 110 is functioning properly. Other modules of network 110, e.g.,reagent input module 152 and mixing modules 152, 166 module 166, areconfigured to accommodate the presence of more than oneamplification/detection chamber.

Various modules of microfluidic network 110 are connected, such as bychannels 164 and 122, to allow materials to be moved from one locationto another within the network 110. Actuators 168, 170, 172 associatedwith the microfluidic device provide a motive force, such as anincreased gas pressure and/or a decreased gas pressure to move thesample and reagent material along the channels and between modules. Somegas actuators move materials by reducing a pressure in a downstreamportion of a microfluidic network relative to a pressure in an upstreamportion of the microfluidic network. The resulting pressure differentialmoves the material downstream toward the region of reduced pressure.Fluid control elements, e.g., valves, gates, vents, and hydrophobicpatches, allow additional control over movement and/or positioning ofthe materials.

As used herein, the term vacuum does not require the total absence ofgas or other material. Rather, a vacuum means a region having at least areduced gas pressure as compared to another region of the microfluidicdevice, e.g., a partial vacuum. The volume of channels and chambersassociated with a vacuum actuator is typically reduced by placing fluidcontrol elements, e.g., valves or gates, as near to the vacuum chamberof the actuator as is feasible.

First actuator 168 of network 110 moves material downstream fromenrichment module 156 to lysing module 158. Upon completion ofprocessing within lysing module 158, a second actuator 170 movesmaterial downstream to DNA clean-up module 160. Subsequently, actuator170 or an additional actuator moves cleaned-up DNA to mixing module 166,where the material mixes with a reagent moved by actuator 172. Finally,actuator 172, or another actuator, moves the mixed material to detectionmodule 162.

Material moved or otherwise manipulated and processed by themicrofluidic device can be in the form of a microdroplet having upstreamand downstream termini typically defined by a liquid gas interface. Insome embodiments, the microdroplets have a volume of 25 μl or less, 10μl or less, 2.5 μl or less, 1 μl or less, 0.5 μl or less, or 0.3 μl orless. Various features of the microfluidic device can be sized toaccommodate such microdroplets. For example, channels and chambers canhave a width of less than 200 μm and a depth of less than 50 μm. Ingeneral, the microdroplets have a length that is substantially shorterthan a length of the channels through which the microdroplets move.

As used herein, the term microfluidic system includes not only amnicrofluidic device defining a microfluidic network but also the heatsources to operate thermally actuated modules, fluid control elements,and actuators of the microfluidic device. The heat sources can beintegrated with the microfluidic device or incorporated in anothercomponent of the microfluidic system such as a receptacle that receivesthe microfluidic device during operation. The various functionalelements, of microfluidic network 110, including the heat sources, aretypically under computer control to allow automatic sample processingand analysis. Systems and methods for computer control of microfluidicsystems are disclosed in U.S. patent application Ser. No. 09/819,105,filed Mar. 28, 2001, which application is incorporated herein byreference.

Control of Microfluidic Systems and Devices

Referring to FIG. 2, an exemplary microfluidic system 99 includes amicrofluidic device 10, a chip carrier cartridge 20, a data acquisitionand control board (“DAQ”) 26, and a processor 27 such as a laptop orpalmtop computer.

Referring also to FIG. 3, microfluidic device 10 has a microfluidicnetwork including microchannels and elements, e.g., valves, pumps,reaction modules, detection modules, and the like, defined using one ormore substrates, which can include, e.g., silicon, glass, polymer, orother suitable material. The microfluidic network can be fabricatedusing photolithography, injection molding, impression molding and othertechniques. Injection molded substrates are typically annealed slowly toprovide a flat substrate that mates with a substrate having heat sourceswith minimal gaps. For example, the injection molded substrate can havea flatness of better than 200 microns, better than 100 microns, orbetter than 50 microns.

System 99 also includes a plurality of components configured to, e.g.,actuate and/or monitor elements of microfluidic device 10. Suchcomponents can include heat sources and temperature sensors. Duringoperation of system 99, heat sources and temperature sensors aretypically disposed within thermal contact of a localized region ofdevice 10. In some embodiments, the components are integral with device10, e.g., the components are fabricated within and/or upon one or moresubstrates that also define the microfluidic network. Alternatively orin combination, the components are fabricated within or upon anotherportion of system 99, e.g., chip carrier cartridge 20. In use, device 10and cartridge 20 mate to bring the heat sources and elements of themicrofluidic network into thermal communication.

Heat sources can be used to control elements such as thermally actuatedfluid control elements, e.g., valves and gates, thermally actuated pumpsand vacuum sources, and reaction chambers. For example, a heat sourcecan melt or otherwise increase a mobility of a thermally responsivesubstance (TRS), e.g., wax, of a thermally actuated fluid controlelement such as a valve or gate thereby allowing the material to moveinto or out of an obstructing position in a channel. Heat sources aretypically in thermal contact with only a localized portion of device 10to the extent that one heat source does not actuate more than oneelement of device 10. System 99 can, however, include heat sources thatare in thermal contact with and simultaneously actuate a selected numberof elements (more than one element) of device 10. Fluid control elementsand TRS's are disclosed in International Application Serial No.PCT/US2004/025181 and in U.S. Pat. No. 6,575,188, issued Jun. 10, 2003,which application and patent are incorporated herein by reference.

A temperature sensor can be used to monitor a temperature of a thermallyactuated element of device 10, e.g., to determine the temperature ofmaterial within a reaction chamber or of a TRS associated with a fluidcontrol element. The temperature sensors generally monitor thetemperature within a localized region, e.g., the temperature of a singleelement, of device 10. System 99 can, however, include temperaturesensors that are in thermal contact with several elements of device 10to simultaneously monitor the temperature of those elements.

In some embodiments, microfluidic device 10 mates with chip carriercartridge 20 to provide a unit that can be inserted into (or removedfrom) an interface hardware receptacle of DAQ 26 having electrical andoptical contacts 25. The mating can position elements of device 10within thermal communication of heat sources and temperature sensors ofcartridge 20. In such embodiments, cartridge 20 generally includes apattern of heat sources and temperature sensors that corresponds to apattern of elements of the microfluidic network of device 10.

The microfluidic device 10 may have electrical and/or optical contacts12, which connect with the chip carrier cartridge for carryingelectrical and optical signals between components of the microfluidicdevice (if disposed thereon) and the cartridge. The contacts and anyleads can be formed with, e.g., wire bonding and photolithographytechniques. In some embodiments, device 10 itself includes contacts thatmate with a receptacle of DAQ 26.

Alternatively or in combination, the chip carrier cartridge 20 may haveelectrical and optical contacts 21 for carrying electrical and opticalsignals between the microfluidic device, the chip cartridge, andcontacts 25 of the data acquisition board 26. For example, the cartridge20 may include components configured to actuate and/or monitor elementsof device 10. Contacts 21 can carry electrical and or optical signalsbetween these components and DAQ board 26.

Some of contacts 12, 21, and/or 25 can be configured for electricalsignals, while others can be configured for optical signals (IR,visible, UV, etc.) in the case of optically-monitored oroptically-excited microfluidic processors. Alternatively (not shown),the entire data acquisition and control board 26 may be a single ASICchip that is incorporated into the chip carrier cartridge 20, whereincontacts 21, 25 may become lines on a printed circuit board.

In general, DAQ 26 allows control of operation of microfluidic device 10via contacts 12, 21, 25 using electrical and optical signals. Processor27 typically performs high level functions, such as supplying a userinterface that allows the user to select desired operations and to viewthe results of such operations. Processor 27 may also include acomputer-readable medium. 29 comprising code to operate device 10 or asystem comprising device 10. As shown in FIG. 2, the processor 27 isconnected to DAQ 26 via connection 128, which provides data I/O, power,ground, reset, and other function connectivity. Processor 27 may also beused to control a laboratory robot 24 via link 31. Alternatively, awireless link 32 between the processor 27 and the DAQ 26 may be providedfor data and control signal exchange via wireless elements 32(a) and32(b). Where the data link is a wireless link, for example, the DAQ 26may have separate power source such as, for example, a battery.

In some embodiments, the number of components, e.g., heat sources and/ortemperature sensors configured to actuate and/or monitor the actuationof various elements of the microfluidic network of device 10, can belarge. If each component required one or more dedicated contacts,contacts between a substrate and a chip cartridge, contacts between achip cartridge and a DAQ receptacle, or contacts between a substrate anda DAQ receptacle, the number of contacts would also be large. Thefollowing description of the operation of a microfluidic device 10 andDAQ 26 demonstrates the relationship between the complexity of themicrofluidic substrate and the requisite number of contacts 12, 21, 25.

FIG. 3 illustrates, schematically and not to scale, the generalstructure of an exemplary integrated microfluidic device. Thismicrofluidic device includes a microfluidic network included includingthree types of sub-assemblies. In particular, this microfluidic networkhas four separate sub-assemblies: two micro-droplet meteringsub-assemblies, metering1 and metering2; one mixing sub-assembly, mixing1; and one reaction/detection sub-assembly, reaction/detection 1.

These sub-assemblies are constructed from elements such as valves,pumps, vents, passages, space to accommodate overflows, reservoirs,inlets, outlets detectors, mixing zones, and the like. For example,sub-assembly metering1 includes inlet1, overflow1, valve1, heater1, andpassage1. Similarly, sub-assembly metering2 includes inlet2, overflow2,valve2, heater2, and passage2. The mixing subassembly, mixing 1,includes heater1, heater2, valve3, valve4, vent1, vent2, Y-shapedpassage3, and passage4. Finally, reaction/detection1 sub-assemblyincludes valve5, valve6, heater3, and passage5.

Some elements of device 10, e.g., valves, pumps, reaction chambers,detection and chambers, can be actuated and/or monitored usingcomponents of system 99. Operations of the sub-assemblies generallyresult from the coordinated operations of their component actuatorsunder the control of an external controller, DAQ 26, which preferablyoperates in accordance with instructions from code of acomputer-readable medium. The specific operation of microfluidic device10 is described in greater detail in co-pending application Ser. No.09/819,105, which is incorporated herein by reference. However, thefollowing describes exemplary operation of the fluid processor under thecontrol of DAQ 26.

First, fluid is introduced into inlet1, for example, by an externalrobotic device, and flows up to the stable position created by the firsthydrophobic region h3 just beyond the widening of passage 1. Any excessfluid flows out through port overflow1. Next, DAQ 26 instructssub-assembly metering1 to measure a micro-droplet of determined volumefrom an aliquot of fluid introduced through port inlet1, as described inco-pending application Ser. No. 09/819,105. Sub-assembly metering2 isconstructed and operates similarly to extract a measured micro-dropletof fluid from a second fluid sample likewise supplied at inlet 2.

After the pair of microdroplets are extracted from the inlet ports, DAQ26 supplies current to heater1 and heater2 to generate gas pressure topropel the two micro-droplets through Y-shaped passage 3 and alongpassage 4 to the stable position in passage 5 just beyond the junctionof the side passage to vent2. During this step, the two microdropletsmerge and mix to form a single, larger micro-droplet.

Next, DAQ 26 supplies current to valve5 and valve6 to close these valvesand isolate the micro-droplet along passage 5. DAQ 26 directs thesub-assembly reaction/detection1 to stimulate a reaction in the trappedmicro-droplet by, for example, supplying current to heater 3, whichheats the micro-droplet. The DAQ then monitors the results of thestimulated reaction by optically detecting radiation conducted byoptical paths o1 and o2. DAQ 26 performs these control functions byselectively supplying electrical (and sometimes optical) signals to themicrofluidic substrate via contacts 12, 21, 25.

DAQ Board Architecture

FIG. 4 illustrates an embodiment of hardware architecture for DAQ board26. The DAQ board has one or more receptacles, slots, or sockets, whereone or more replaceable microfluidic devices may be accommodated in afirmly supporting manner with good contact to its external contacts. Insome embodiments, the DAQ board accommodates or includes a chipcartridge including a plurality of heat sources. The microfluidic devicemates with the chip cartridge to place the heat sources and thermallyactuated components of the microfluidic device in thermal communication.

As shown, electrical contacts 25(a) on the DAQ mate with correspondingcontacts 21(a) of the chip carrier cartridge 20. Thus, leads 39, 40 ofthe DAQ are electrically connected to corresponding leads of the chipcarrier cartridge 20. Similarly, contacts 25(b) of the DAQ mate withcontacts 21(b) of the chip carrier cartridge, thereby connecting vialight pipe, line of sight, or by other suitable means, the DAQ's opticalcouplings 41,42 to corresponding optical couplings on the chip carriercartridge. The electrical and optical leads of the chip carriercartridge are, in turn, connected to the microfluidic device 10 viacontacts 12. Thus, DAQ 26 can send and receive electrical and opticalsignals via contacts 12, 21, 25 to and from microfluidic device 10 inorder to engage and control a variety of components or actuators locatedon cartridge 20 and/or device 10.

The electrical contacts, which may have many embodiments, areillustrated here as edge contacts that are engaged when the chip carrierand microfluidic substrate are inserted in a DAQ board receptacle.Alternatively, contacts may be suitable for engaging a flexible ribboncable, or may by multi-pin sockets, for example. The optical contactsmay be of types known for connecting fiber-optic cables.

The DAQ may include one or more electrical energy sources such as heatsource drivers 47 for supplying a specified amount of current and/or aparticular voltage. The output of each heat source driver 47 may beconnected to an analog multiplexer 48 that routes the current from thedriver to a selected I/O contact 25(a). For thermal sensing functions,the DAQ may include one or more electrical energy sources such astemperature sensor drivers 49 which are each connected to an analogmultiplexer 50 that multiplexes each temperature sensor driver 49 to aselected one of the plurality of I/O contacts 25(a).

In some embodiments, heating and sensing functions are provided by asingle element, such as a component with temperature dependentresistance. Such combined elements can be operated by an electricalenergy source, e.g., a voltage or current supply. Typically anelectrical energy source is configured, in a first actuation state, toprovide current or electrical potential sufficient to heat the resistivecomponent. The energy to heat the resistive component is generallysufficient to heat an element (e.g., a TRS and/or pressure chamber of avalve, a TRS of a gate, reaction chamber contents, or a gas actuatedpump) of microfluidic device 10 in thermal contact with the resistivecomponent. In a second activation actuation state, the electrical energysource provides a current or electrical potential sufficient to operatea sensing function of the component. The actuation state of the one ormore electrical energy sources driving a combined heating sensingcomponent may be determined by code of a computer readable medium.

The DAQ 26 can include one or more photodiodes 51 for optical detection.Multiplexor 52 multiplexes signals from and to these optical detectorsto an analog-to digital converter (“ADC”) 55 via a selected one of theplurality of I/O contacts 25(b). Finally, the DAQ is shown ascontrolling one or more laser diodes 53. Laser enable register 54enables selected laser diode drivers, thereby emitting light signals oncorresponding optical couplings 42 and optical contacts 25(b).

Also shown in FIG. 4, the DAQ also includes a microprocessor and memory43 for controlling the operation of the drivers 47, temperature sensordrivers 49, photodiodes 51, laser diodes 53 and their associated analogmultiplexors 48, 50, 52, as well as laser enable register 54. Morespecifically, the microprocessor sends control signals to these devicesvia a bus driver 45 and bus 46, and reads status information from thesensing elements via the same driver 45 and bus 46. Finally, hostinterface 44 allows the microprocessor 43 to communicate with thegeneral purpose processor 27 (FIG. 2) via leads 38(c) or, as describedabove, via wireless means.

The operation of the DAQ is exemplified by the following description ofthe control of a simple resistive heat source, such as the resistiveheater shown in valve 1 of the microfluidic device depicted in FIG. 3.As shown in FIG. 3, valve1 includes a resistive heating element 9 thatis connected at its terminals 11, 13 to a pair of I/O contacts 12(a) vialeads 8. The DAQ activates this resistive heating element by instructinganalog multiplexor 48 to connect the output of heat source driver 47 toa pair of I/O contacts 25(a) that are connected to corresponding I/Ocontacts 21(a) of the chip carrier 20, that are connected tocorresponding contacts 12(a) of the substrate. It then instructs heatsource driver 47 to supply a selected amount of current. The currentsupplied by driver 47 flows through analog multiplexor 48 and to theresistive heating element 9 via the selected leads 39 and 8.

Referring to FIGS. 5A and 5B, an exemplary operation of a fluid controlelement, e.g., a valve, is described. FIG. 5A depicts the valve in itsopen position, having a mass of TRS, e.g., a wax plug 76, positionedwithin side channel 77. The mass of TRS can have a volume of about 250nanoliters or less, about 125 nl or less, e.g., about 75 nl or less. Toclose this valve, DAQ controller supplies current to resistive heaterHTR2 via I/O contacts 80, 81. This causes HTR2 to warm, thereby meltingplug 76. DAQ 26 then supplies current to HTR1 via I/O contacts 82, 84 tothereby heat gas within chamber 75. As the gas expands, it forces plug76 to move into channel 78 as shown in FIG. 4B. DAQ 26 then shuts offheater HTR2 and allows the plug to cool, thereby blocking channel 78 andside channel 77. When the plug is cool, DAQ 26 shuts off HTR1. As HTR1cools, the pressure in chamber 75 drops, thereby creating a negativepressure which, as will be explained below, may be used to re-open thevalve.

To open the valve, DAQ 26 supplies current to HTR3 via I/O pins 86, 88to warm the heater and thereby melt the plug. Once the plug is melted,the negative pressure in chamber 75 draws the plug back into sidechannel 77, thereby re-opening channel 78.

In some embodiments of a fluid control element, some or all of the massof TRS moves downstream upon opening the control element. Such fluidcontrol elements are referred to as gates.

FIGS. 6A and 6B-6D depict heating and sensing components suitable foruse in microfluidic devices, such as to open or close valves, e.g., asdiscussed with respect to FIGS. 5A/5B, heat reaction mixtures present inreaction chambers, or provide a material transport function, such as byactuating a thermally actuated pump.

FIG. 6A depicts a six-terminal resistive heating and sensing component.The component includes a two terminal heat source R1 that operates inaccordance with resistive heating element 9 of FIG. 3. The device alsoincludes a current flow directional element 70, which allows current toflow substantially only in a single direction between leads 55, 56 ofheat source R1. As shown in FIG. 6A, current flow directional element 70is a diode configured to allow current to flow from lead 56 to lead 55.Current flow directional element 70 substantially prevents, andpreferably excludes, current flow from lead 55 to lead 56. Current flowdirectional element 70 may be any element that allows current to flowpredominately in one direction between points of a circuit. Current flowdirectional elements are typically diodes.

The device of FIG. 6A also includes a four terminal temperature sensor,e.g., a resistive sensor component R2 in close proximity to R1 so as tobe in thermal communication therewith. A current flow directionalelement 71, which has the generally the same functional characteristicsas current flow directional element 70, allows current to flow insubstantially one direction between leads 57, 58 and leads 59, 60 ofresistive sensor component R2. In the configuration shown, current flowdirectional element 71 allows current to flow from leads 59 and 60 toleads 57 and 58 but substantially prevents, and preferably excludes,current flow from leads 57 and 58 to leads 59 and 60.

Current flow directional elements 70 and 71 may be but are notnecessarily formed by microfabrication on a substrate with elements R1and R2. Rather, current flow directional elements 70 and 71 may bedisposed at other positions along current pathways that respectivelyinclude R1 and R2. Current flow directional elements 70 and 71 aregenerally disposed in series with R1 and R2.

The sensor R2 may operate as follows. While DAQ 26 supplies current toR1 (via leads 55, 56) it also supplies a relatively low current to R2via leads 57, 60. R2 is a resistive component whose resistance generallyincreases with temperature. Accordingly, the voltage across R2 increaseswith the temperature in the nearby region being heated by heat sourceR1. Therefore, component R2 can be used to measure the temperature inthis region. DAQ 26 determines the temperature by measuring the voltageacross R2 via leads 58, 59. More specifically, referring now to FIG. 4,DAQ 26 instructs the analog multiplexor to connect temperature sensordriver 49 to the contact pins 25(a) which are connected to leads 58, 59.Sensor Temperature sensor driver 49 then determines the voltage acrossR2, thereby providing a measure of the temperature in the vicinity ofR1.

The Relationship Between the Number of I/O Contacts and the Number ofComponents Actuating and/or Monitoring Elements of the MicrofluidicSystem

For a two terminal component, such as the resistive heater R1 describedabove, the system may use two I/O contacts to supply the control signalsfor operation of the component. Thus, if the number of two-terminalcomponents of system 99 is N, then 2N I/O contacts are sufficient toallow DAQ 26 to independently control each of the components.

However, for complex microfluidic devices, the number of I/O contactscan become large. In the microfluidic device shown in FIG. 3, where onlynine different resistive heat sources are shown, eighteen contacts arerequired. For increasingly complex microfluidic devices having hundredsof independently controlled components, the number of contacts becomeslarger.

Techniques for reducing the number of I/O contacts required for anexternal controller, such as DAQ 26, to independently control a largenumber of components of a microfluidic device are discussed below.

Combined Heating Temperature Sensing Components

Referring to FIGS. 6B and 6C, a combined heating/temperature sensingcomponent (CHTSC) 350 is operable both as a heat source and as atemperature sensor. Component 350 is shown in thermal communication witha thermally actuated component of a microfluidic device 352, only aportion of which is shown. FIGS. 6B and 6C do not illustrate othercomponents, such as other microfluidic elements, modules, passages,pumps, valves, reaction chambers, access ports, and the like thatmicrofluidic device 352 may include.

The thermally actuated component is, by way of an example only, areaction chamber 360 having an upstream channel 361 and a downstreamchannel 363. It should be understood that such thermally actuatedcomponents are not limited to reaction chambers but can include, e.g.,pressure actuators or fluid control elements.

Device 352 includes a substrate 370, which typically includes first andsecond substrate portions 371, 373 defining a microfluidic networkincluding reaction chamber 360 therebetween. Device 352 can also includea third substrate portion 375. Exemplary microfluidic devices arediscussed in International Application Serial No. PCT/US2004/025181.

Combined heating/temperature sensing component (CHTSC) 350 includes acomponent R1′, which, in a first actuation state, generates thermalenergy to maintain or increase a temperature of the microfluidicelement, e.g., material within chamber 360, and, in a second actuationstate, may be used to obtain electrical property data indicative of atemperature of component R1′. Exemplary components R1′ includetemperature-dependent resistors and elements including junctions betweentwo dissimilar materials. The device 352 also includes leads 355 and 356by which electrical energy may be supplied to element R1′. Leads 355,356 and element R1′ form an electrical pathway comprising element R1′,which is disposed in thermal communication with the microfluidic elementreaction chamber 360. An electrical energy source may be used to placean electrical potential across element R1′ via leads 355, 356.

Device 10 or a system configured to operate device 10 may include anelectrical measurement device, for example, an ammeter, configured toprovide data indicative of an electrical characteristic, such as acurrent, power dissipation, or electrical potential drop, of theresistive component.

Referring to FIG. 6D, an exemplary operation of combined/heater sensorR1′ is illustrated using a plot of power dissipation by R1′ as afunction of time. Power dissipation is indicative of the amount ofenergy dissipated per unit time by the CHTSC. The heating/temperaturesensing component can be operated under the control of code of acomputer-readable medium.

In use, microfluidic devices and/or microfluidic systems comprisingCHTSC components typically include one or more electrical energy sourcesin electrical communication with the CHTSC. The system includes acomputer-readable medium having code to operate the one or moreelectrical energy sources. For example, the computer readable medium mayinclude code to provide first and second actuation states of anelectrical energy source in electrical communication with the CHTSC.During the first actuation state, a first electrical current flowsthrough the CHTSC, e.g., through a resistive component or dissimilarmetal junction thereof. During the second actuation state, a second,lower electrical current flows through the CITFSC.

During the second actuation state, current can be supplied to the CHISCto determine an electrical property thereof, e.g., a resistance thereof.The current during the second actuation state does not heat the CHTSC toa temperature exceeding the temperature of the CHTSC immediately priorto initiating the second actuation state. The voltage across the CHTSC,produced by the current, can be sampled, such as by a traditional sampleand hold amplifier, which may be configured to sample only during thesecond actuation state. The output of the sample and hold amplifier maythen be fed to an analog to digital converter which may generatetemperature data used for system feedback and control.

The CHTSC 350 and microfabricated element 360 component R1′ may becharacterized by a dissipation constant (DC) having units of power perdegree, for example watt/° C. If a microfabricated device comprising aCHTSC of the invention (a) is at an ambient temperature of about 20° C.and (b) the CHTSC dissipates an amount of power k, the temperature of atleast a portion of the microfluidic element in thermal contact with theCHTSC typically rises to and/or is maintained at a temperature T=k/DC.For example, given sufficient time, e.g., about 60 seconds, a CHTSC inthermal communication with a reaction chamber, typically heatssubstantially all of the material, e.g., a PCR mixture or othermaterials, in the chamber to a temperature k/DC. Given sufficient time,e.g., about 60 seconds, a CHTSC in thermal communication with athermally responsive material, e.g. a wax, of a valve, typically heatssubstantially all of the material to a temperature k/DC.

It should be understood that the duration of a single first actuationstate may be insufficient to raise the temperature of the microfluidicelement to the temperature k/DC. Repeated first actuation states may beperformed. In general, first actuation states are separated by secondactuation states, during which, a temperature sensing function isperformed.

During a first actuation state, the CHTSC dissipates an amount of powerk1. The ratio k1/DC is typically about 30° C. or more, such as about 50°C. or more, about 55° C. or more, for example, about 60° C. or more, orabout 65° C. or more. The ratio k1/DC may be about 100° C. or less, suchabout 95° C. or less, for example, about 85° C. or less.

In one embodiment, sufficient power is dissipated during the firstactuation state (or upon repeated applications of the first actuationstate) that the temperature of liquids within a microfluidic reactionchamber in thermal contact with the CHTSC would rise to a temperaturesufficient to support amplification of polynucleotides present in theliquids. The first actuation state (or the repeated first actuationstates) may generate sufficient thermal energy to heat at least theportion of the microfluidic element in thermal contact with the CHTSC toabout 30° C. or more, such as about 50° C. or more, about 55° C. ormore, for example, about 60° C. or more, or about 65° C. or more. In thefirst actuation state (or the repeated applications thereof), theportion of the microfluidic element in thermal contact with the CITSC isgenerally heated to less more than 200° C., less than 150° C., less than100° C., less than 95° C., for example, less than 85° C.

During a second actuation state of the CHTSC, the component maydissipate an amount of power k2, which is preferably smaller than k1.For example, if the second actuation state follows the first actuationstate in time, the temperature of the CHTSC and or the microfluidicelement in thermal contact therewith may fall during second actuationstate from a temperature attained during the first actuation state. Ifthe first actuation state follows a second actuation state, thetemperature may rise during the first actuation state as compared to thetemperature immediately preceding the initiation of the first actuationstate. The ratio k2/DC is preferably about 50° C. or less, such as about45° C. or less, for example, about 40° C. or less, about 35° C. or less,or about 30° C. or less.

As shown in FIG. 6D, the code may operate the electrical energy sourcein the second actuation state for a time Δτ2. The code may operate theelectrical energy source in the first actuation state for a time Δτ1.The power dissipated need not be constant during each actuation state.The power dissipated need not be the same between successive firstactuation states or successive second actuation states. In oneembodiment, the amount of power dissipated during the second actuationstate and the duration of the second actuation state are such that theabsolute temperature of the microfluidic element in thermal contact withthe CHTSC falls by less than 5%, for example less than 2.5%, of themaximum temperature reached during a preceding first actuation state.For example, the absolute temperature of a reaction mixture in areaction chamber in thermal contact with a CHTSC may fall by less than5%, for example less than 2.5%, of the maximum temperature reachedduring a preceding first actuation state.

The lengths of the first and second actuation states may be different.The lengths of successive first actuation states and successive secondactuation states may be different. In some embodiments, first actuationstates are shorter than 60 seconds, shorter than 45 seconds, shorterthan 15 seconds, shorter than 1 second. Second actuation states may bethe same length as first actuation states or shorter. For example,second actuation states may be 5 seconds or less, 1 second or less, 0.1seconds or less, or 0.01 seconds or less.

In some embodiments, the second actuation states have a duty cycle of nomore than about 5%, no more than about 2.5%, no more than about 0.5%, nomore than about 0.25%, or no more than about 0.2% and the firstactuation states have a duty cycle of no more than 90%, no more than95%, no more than 97.5%, or no more than 99%. During a remaining portionof the duty cycle, if any, the system is neither in the first nor thesecond duty cycle. This remaining portion may be used to stabilize acircuit that determines an electrical property of the heater/sensor,e.g., a voltage drop thereacross. For example, this circuit may includea diode clamp that is stabilized during the remaining portion of theduty cycle. In some embodiments, during the remaining portion, nocurrent is passed through the heater sensor.

The computer-readable medium typically includes code to sense anelectrical property of the CHTSC, determine a temperature from thesensed property, compare the temperature to a selected temperature, anduse the result of the comparison in a feedback loop to adjust furtherheating and temperature sensing steps. Examples of such code arediscussed below.

The computer-readable medium may include code to receive data indicativeof an electrical property of the CHTSC, such as of a resistive componentthereof, from the electrical measurement device. The data indicative ofthe electrical characteristic of the resistive component may beindicative of a temperature-dependent resistance of a resistivecomponent of the CHTSC. In some embodiments, the data indicative of theelectrical characteristic of the resistive component is indicative of anelectrical potential required to cause a predetermined current to flowthrough the CHTSC. The data indicative of the temperature-dependentelectrical characteristic of the CHTSC may be obtained while theelectrical energy source is in the second actuation state.

The computer-readable medium may comprise code to determine atemperature of the resistive component based on the data indicative ofthe electrical property. The data may be indicative of the electricalcharacteristic of the resistive component when the electrical energysource is in the second actuation state. There may be code to comparethe temperature of the resistive component with a predeterminedtemperature value, and, optionally, code to repeat the first and secondactuation states of the electrical energy source if the temperature isless than the predetermined temperature value. The medium may comprisecode to repeatedly determine the temperature of the CHTSC, compare thetemperature of the CHTSC with the predetermined temperature value, andrepeat the first and second actuation states of the electrical energysource, the determination of temperature, and the comparison oftemperature and the predetermined temperature value if the temperatureis less than the predetermined temperature value. The computer-readablemedium may comprise code to vary, at least once, at least one of thefirst and second currents when repeating the first and second actuationstates of the electrical energy source.

The computer-readable medium can include code to compare, based upon thereceived data indicative of the electrical characteristic, (i) a currentflowing through the CHTSC, e.g., during the second actuation state, and(ii) a predetermined current. The medium can include code to increase anelectrical potential across a portion of the CHTSC, such as a resistivecomponent thereof, during the second actuation state if the second,lower current is less than the predetermined current. There may be codeto decrease an electrical potential across the CHTSC during the secondactuation state if the second, lower current exceeds the predeterminedcurrent. There may be code to receive electrical potential data (e.g.,from an electrical energy source) indicative of the electrical potentialacross the CHTSC during the second actuation state if the second, lowercurrent is within a predetermined range of the predetermined current.The computer-readable medium may comprise code to determine thetemperature of the CHTSC based on the electrical potential across theCHTSC when the second, lower current is within the predetermined rangeof the predetermined current.

The computer-readable medium may comprise code to provide the firstactuation state of the electrical energy source if the temperature ofthe resistive component is less than a predetermined temperature. Thecomputer-readable medium may comprise code to repeatedly determine thetemperature of the CHTSC, such as a resistive component thereof, basedon the electrical potential across the CHTSC when the second, lowercurrent is within the predetermined range of the predetermined currentand provide the first actuation state of the electrical energy source ifthe temperature of the CHTSC is less than the predetermined temperature.

Multiplexed Actuation of Heat Sources and Other Components

FIGS. 7A, 7B illustrate a technique for reducing the number of I/Ocontacts by structuring the leads that provide current to the heatsources, temperature sensors, or combined heat source/sensors of themicrofluidic device so that each lead serves more than one component,while still allowing DAQ 26 to control each thermally actuated componentof the microfluidic device independently of others. Specifically, FIGS.7A, 7B depict a technique for sharing I/O contacts among three of thetwo-terminal resistors of a valve structure, such as shown in FIGS.5A-5B discussed above. The valve operates essentially the same as thevalve shown in. FIGS. 5A, 5B, except that it uses only four contactsrather than six. In this example, each resistor is connected to a pairof I/O contacts and therefore can be controlled by the DAQ in the sameway as described above. Although the other resistors share these I/Ocontacts, no resistor shares the same pair of contacts with another.Accordingly, the DAQ is able to supply current to any given resistor viathe pair of associated contacts, without activating any other resistor.

More generally, the number of I/O contacts required for the independentcontrol of a plurality of heat sources, e.g., resistive heaters, may bereduced by arranging the contact wiring to each resistor in the form ofa logical array. The resulting compression of the number of I/O contactsadvantageously simplifies communication with the entire processor.Because each resistor requires two leads to complete an electricalcircuit, according to a conventional arrangement of leads and contacts,a device having N resistors requires 2N leads and 2N contacts. Byconfiguring the contact wiring in a shared array, however, the number ofrequired contacts can be reduced to as few as 2N^(1/2). For example, ina device comprising 100 resistors, the number of external contacts canbe reduced from 200 to 20.

FIGS. 8A, 8B depict a DAQ 26 directly connected to a microfluidicsubstrate 22, without the use of an intermediate chip carrier cartridge20, and show an array of resistive heaters within substrate 22. Theleads between contacts 12(a) and resistive heaters 100, 102-109 areshown arranged in columns and rows. However, the actual physical layoutof the leads will not necessarily be a physical array. Rather, the leadsmay be directly routed from the resistive components to contacts 12(a)in any manner that allows each lead to connect to a plurality ofresistors while remaining electrically isolated from other leads.

According to this arrangement, electrical contacts for N resistors canbe assigned to R rows and C columns such that the product RC is greaterthan or equal to N. Typically, R is approximately equal to C, andgenerally equals C. With this arrangement, resistors assigned to thesame row share a common electrical lead and I/O contact 12(a).Similarly, resistors assigned to the same column also share a lead andI/O contact 12(a). However, each resistor has a unique address,corresponding to a unique pair of I/O contacts, (e.g., to its uniquerow/column combination in the array). Therefore, each resistor isindividually actuatable by supplying electric current to the appropriatepair of I/O contacts.

As used herein, a “resistor” or “component” that is uniquely associatedwith a pair of contacts may also refer to a resistive network (having aplurality of resistive sub-components contacted in series and/orparallel) or a component network (having a plurality of sub-componentsconnected in series or parallel). In such embodiments, allsub-components are activated together when the external controllersupplies signals across the pair of contacts uniquely associated withthose sub-components.

As shown in FIG. 8A, the leads are arranged in three rows (Rj, where byway of example j=1-3) and three columns (Ci, where by way of examplei=1-3). For each resistor, one terminal is connected to a row and theother terminal is connected to a column. Although each resistor sharesthese leads with other resistors, no two resistors share the same pairof leads. In other words, each resistor is uniquely associated with aparticular row/column pair Rj, Ci. FIGS. 8A, 8B illustrate the operationof this structure. Heat source driver 47 supplies an output voltage oftwenty volts on its terminals for supplying current to resistive heaters100, 102-109. The positive output terminal 90 is connected to a firstanalog multiplexor 48(a). As shown, this terminal can be connected toany one of the rows of the array of leads by individual switchingelements within analog multiplexor 48(a). Similarly, the negative outputterminal 92 of heat source driver 47 is connected to a second analogmultiplexor 48(b). Multiplexer 48(b) allows terminal 92 to connect toany column in the array of leads.

In FIG. 8A, the switching elements within analog multiplexors 48(a,b)are all open. Accordingly, none of the resistive heaters 100, 102-109 asshown are active. FIG. 8B depicts the condition of analog multiplexors48(a,b) after DAQ 26 has instructed them to close certain internalswitches to thereby supply current to a selected one of the resistors inthe array. In this example, the row switch element 50 is closed, tothereby connect the positive terminal of heat source driver 47 to thetop row of the lead array. The column switch element 52 is also closedto connect the negative terminal of heat source driver 47 to the middlecolumn of the lead array. Thus, the positive terminal 90 of heat sourcedriver 47 is connected to resistive heaters 100,102,103 and the negativeterminal is connected to resistive heaters 102,105,108. However, onlyone of these resistive heaters, 102, is connected across both terminalsof heat source driver 47. Accordingly only resistive heater 102 receivescurrent and is heated.

Resistive heaters 100, 102-109 are disposed in series with respectivecurrent flow directional elements 215-223, which allow current to flowin one direction between the positive terminal 90 of a heat sourcedriver 47 and a negative or ground terminal 92 of heat source driver 47along a current path that includes one of resistive heaters 100,102-109. Current flow directional elements 215-223 are typicallyconfigured to allow current to flow only from positive terminal 90 toterminal 92. Thus, for example, current may flow from a point 224 to apoint 225, through resistive heater 102 to point 226 and then to point227. The current flow directional elements, however, prevent currentfrom passing through current pathways including resistive heaters otherthan resistive heater 102. For example, current flow directional element219 prevents current flow between points 228 and 229. Current flowdirectional elements 215-223 may be diodes as discussed above forcurrent flow directional elements 70, 71.

FIGS. 9A, 9B, 10A, 10B depict similar arrays for the resistivecomponents used to sense temperature, such as R2 shown in FIG. 6B. FIG.9A depicts one array of leads for supplying current to resistive sensors101, 111-118. FIG. 9B depicts another set of leads for measuring thevoltage across the same resistors. With this structure, the leads thatare used to stimulate the resistive sensors carry no current from theheat source driver 47 because they are electrically isolated from driver47. Similarly, the leads for sensing the voltage of the resistivesensors 101, 111-118 (FIG. 9B) carry essentially no current because theyare isolated from the leads that supply current from drivers 47 and49(a) (shown in FIGS. 8A, 8B and 9A).

FIGS. 10A, 10B depict an alternative structure. As with the structureshown in FIGS. 9A, 9B, the leads for sensing the voltage acrossresistive sensors 101, 111-118, are isolated from both of the currentsources (heat source driver 47 and RTD driver 49(a)). However, bothcurrent sources 47, 49(a) share the same leads for current return, i.e.,the leads depicted as columns in the array. This provides greatercompression of the number of leads; however, the resistivity in theshared return leads may reduce the accuracy of the temperaturemeasurement.

The arrays of FIGS. 9A, 9B, 10A, and 10B include current flowdirectional elements 215′-223′, which allow current to flow in only onedirection through resistive sensors 101, 111-118. Thus, current flowdirectional elements 215′-223′ preferably allow current to flow in onlyone direction between the positive terminal of RTD drive or RTD senseand the negative or ground terminal of RTD drive or RTD sense along acurrent path that includes one of resistive sensors 101, 111-118.Typically, current flow directional elements 215′-223′ allow current toflow from the positive terminal to the negative terminal or groundterminal of either RTD drive or RTD sense but not from the negative orground terminal to the positive terminal thereof. Current flowdirectional elements 215′-223′ may be diodes similar to current flowdirectional elements 70, 71.

Components Having a Plurality of Heat Sources

One technique that typically reduces the number of contacts required tooperate a plurality of heat sources is to integrate multiple heatsources into a single components.

Referring to FIG. 11, a substrate 3200 includes a conductor 3201defining a first end 3202 and a second end 3204. Between the first andsecond ends, conductor 3201 includes a plurality of connective regions3206 having a first conductivity and a plurality of active regions 3208having a second, lower conductivity. The various connective and activeregions of conductor 3201 are connected in series, with consecutiveactive regions 3208 spaced apart by a connective region 3206. Conductor3201 and consecutive connective regions 3206 thereof define a majorlongitudinal axis, which intersects at least a plurality of the regions3206 and 3208.

Substrate 3200 typically has a lower electrical and thermal conductivitythan conductor 3201, e.g., the substrate may be non-conductive.Substrate 3200 is typically fabricated from materials including, e.g.,silicon, various oxides, organic compounds, quartz, glass, polymers,polyamide, polyimide, imide-triazine, glass-epoxy, and combinationsthereof. In some embodiments, substrate 3200 is fabricated of materialstypically used in printed circuit boards, e.g., a polyimide wafer. Insome embodiments, the electrical leads supplying current to the heatsources are fabricated on the wafer so that there is no change insubstrate between the interconnects to the power supplies and DAQ theheat sources. In some embodiments, substrate 3200 is fabricated ofmaterials more flexible than quartz or silicon.

Typically, conductor 3201 is formed of metal deposited upon substrate3200. For example, a metal foil about 2 mils thick can be bonded to thesubstrate and etched to prepare a pattern of heat sources andconductors. In some embodiments, some or all of conductor 3201 issandwiched between layers of substrate 3200. For example, conductor 3201may be covered by a non-conductive coating such as an oxide or polymercoating. In other embodiments, conductor 3201 is covered by a layerhaving an isotropic thermal conductivity. Typically, the thermalconductivity is highest normal to the surface of substrate 3200 andlower in the plane of the surface.

Active regions 3208 typically have a higher resistance than, connectiveregions 3206. Thus, current flowing through conductor 3201 dissipatesmore heat, within active regions 3208 than within connective regions3206 so that each active region 3208 operates as a heat source. Theamount of heat generated in different active regions may be different sothat one active region dissipates more heat than another active regionwhen current flows through conductor 3201. In some embodiments, theactive regions have a smaller cross sectional area taken along adimension generally perpendicular to a current pathway through theconductor.

Alternatively or in combination with varying the cross sectional area ofthe active and conductive regions, conductivity differences can beachieved by incorporating different materials within each type ofregion. In some embodiments, the active regions include a junctionbetween dissimilar materials, e.g., different metals. Passage of currentthrough the junction generates heat. In some embodiments, passage ofcurrent through the junction reduces the temperature of the junction andprovides a cooling effect. In general, however, the active andconnective regions are formed of the same material.

Substrate 3200 also includes a second conductor 3211 defining a firstend 3212 and a second end 3214. Between the first and second ends,conductor 3211 includes a plurality of connective regions 3216 having afirst conductivity and a plurality of active regions 3218 typicallyhaving a second, lower conductivity. The various connective and activeregions of conductor 3211 are connected in series, with consecutiveactive regions 3218 spaced apart by a connective region 3216.Consecutive connective regions 3216 generally (but not necessarily)define a major longitudinal axis. At least some of the active regions3218 are disposed laterally to the longitudinal axis of consecutiveconnective regions. In other respects, however, connective and activeregions of conductor 3211 may be identical with connective and activeregions of conductor 3201.

Substrate 3200 also includes heat sources 3339, 3341, and 3343, whichare not multiplexed. Thus, passing current through these heat sourcestypically generates heat within a single locality of substrate 3200.

Substrate 3200 is received by (or is integral with) a chip carriercartridge 3345, similar to chip carrier cartridge 20. Cartridge 3345includes connections 3349 that allow electrical and other signals to beinput to and received from substrate 3200. Connections 3349 aregenerally in communication with a DAQ configured to operate heat sourcesof substrate 3200. Cartridge 3345 is typically a PCB and can beconfigured to receive a plurality of different substrates 3200, eachhaving a different pattern of heat sources configured to actuatecomponents of a different microfluidic network.

In use, substrate 3200 mates with a microfluidic device 3220 comprisinga plurality of layers defining a microfluidic network 3222 therebetween.Typically, device 3220 and substrate 3200 mate with a precision ofbetter than 100 microns in dimensions parallel to the planes of thesubstrate and device.

Network 3222 includes an input module 3300, an enrichment module 3302, alysing module 3304, an actuator 3306, a DNA clean-up module 3308, aplurality of fluid control elements 3310, and a plurality ofreaction-detection chambers 3312. Various components of network 3222 areconnected by channels 3314. Various microfluidic devices and fabricationtechniques are discussed in International Application Serial No.PCT/US/2004/025181.

When device 3220 is mated with substrate 3200, active regions 3208 and3218 are disposed in thermal communication with thermally actuatedelements of the microfluidic network. For example, active regions 3208thermally communicate with reaction-detection modules 3312 and activeregions 3218 thermally communicate with fluid control elements 3310.

Typically, heat sources (active regions) of substrate 3200 in thermalcommunication with valves and gates of device 3220 are configured toheat an area of device 3220 that is somewhat larger than the areaoccupied by TRS of the valves or gates.

Whether for a gate or a valve, the obstructing mass of TRS can have avolume of 250 nl or less, 125 nl or less, 75 nl or less, 50 nl or less,25, nl or less, 10 nl or less, 2.5 nl or less, 1 nl or less, e.g., 750pico liters or less. In some embodiments of a gate or valve, some or allof the TRS passes downstream upon opening the gate or valve. Forexample, the TRS may pass downstream along the same channel as samplepreviously obstructed by the TRS. In some embodiments, the TRS melts andcoats walls of the channel downstream from the position occupied by theTRS in the closed state. The walls may be at least partially coated forseveral mm downstream. In some embodiments, the TRS disperses and passesdownstream as particles too small to obstruct the channel.

Upon passing a current through conductor 3201, active regions 3208typically dissipate an amount of heat proportional to the magnitude ofthe current and to the resistance of the active region. The heat raisesthe temperature of material within reaction-detection modules 3312. Insome embodiments, a respective polymerase chain reaction mixture presentin each module 3312 can be heated and allowed to cool repeatedly toallow amplification of DNA present therein. In some embodiments, device3220 includes a plurality of lysing modules that mate with differentactive regions of a conductor. The active regions generate heat to lysecells present in respective lysing modules. Reaction-detection andlysing modules are disclosed in International Application Serial No.PCT/US2004/025181.

Fluid control elements 3310 are valves or gates that selectivelyobstruct passage (in a closed state) or allow passage (in an open state)of material along channel 3314 between clean-up module 3308 andreaction-detection modules 3312. Passing current through conductor 3211simultaneously actuates all of the elements 3310.

In some embodiments, the actuation simultaneously opens some valves andcloses others. In other embodiments, all of the valves aresimultaneously actuated from one state to another state.

Active regions of a single conductor can be actuated to, e.g., closemultiple valves simultaneously, open multiple gates simultaneously, openmultiple gates and close multiple valves simultaneously, heat multiplereaction chambers to simultaneously perform multiple reactions, e.g.,isothermal reactions, thermocycling, simultaneously, or generatepressures and/or vacuums in multiple on-chip pumps simultaneously. Insome embodiments, the thermally actuated elements associated with theactive regions of conductor 3201 include a combination of componentsselected from the group including, e.g., valves, gates, reactionchambers, pressure sources, vacuum sources, and the like.

Conductive regions 3206 generally do not dissipate sufficient heat toactuate thermally actuated components of the device or cause evaporationof liquids within the microfluidic network. For example, the temperaturegenerated within device 3220 may drop by 30° C. or more, by 50° C. ormore, or by 75° or more between consecutive active regions. Consecutiveactive regions can be spaced apart by, e.g., 2 cm or less, 1 cm or less,or 5 mm or less.

In general, each active region heats only a localized region of a givenmicrofluidic network so that the heat generated by an active region issufficient to actuate only a single element of the microfluidic network.

Rather than having a single microfluidic network, a microfluidic devicecan include a plurality of separate microfluidic networks. Each networkis configured to process a different sample and/or control. In someembodiments, different networks are configured to perform a differentfunction, e.g., sample enrichment, cellular lysing, sample cleanup,detection, polynucleotide amplification, and the like. The differentnetworks can be located within a single substrate.

Active regions connected in series, as are 3208 and 3218, typically passan identical amount of current. Active regions can also be connected inparallel with one another. A first active region in parallel with asecond active region generally passes an amount of current related tothe magnitude of the current passing through the conductor and the ratioof the resistance of the first active region to the total resistance ofthe first and second active regions.

Exemplary techniques for fabricating substrates such as substrate 3200are discussed with respect to FIGS. 12 a and 12 b in which a substrate4300 defines a generally flat upper surface 4301 and a lower surface4303. Upper surface 4301 includes a plurality of active regions 4302(e.g., heat elements, temperature sensors, or combinations thereof) onlyone of which is shown.

Conductive vias 4305 typically pass partially or completely throughsubstrate 4300 so that leads 4306 that supply current to active regions4302 are spaced apart from an, upper surface 4301 by at least a portionof substrate 4300. Thus, leads 4306 are generally buried withinsubstrate 4300 and/or run along lower surface 4303. Use of vias toprovide active regions with current allows the density of active regionsto be increased compared to fabricating both leads and active regions onthe same surface of the substrate.

Methods for creating substrate 4300 include flip chip techniques.Generally, an organic substrate such as a PCB is cut into a desiredshape, e.g., rectangular or circular. Flats to facilitate handling ofthe substrate in semiconductor processing equipment may be provided.Holes are formed to accommodate vias and the vias formed using standardcircuit board techniques. Connective regions (leads) are applied, e.g.,to the back side of the substrate, to provide electrical contact betweenthe vias of different active regions. The connective regions aregenerally copper interconnects or wires having a minimal width of about35 microns to 100 microns and a thickness of about 12.5 microns to about35 microns.

The upper surface of the substrate is polished until smooth. In someembodiments, the surface roughness is reduced to less than about 15%,e.g., less than about 10% of the active regions to be applied. In someembodiments, the top surface is polished to create a smooth surface,e.g., a surface finish of SPI A1/SPI A2/SPI A3, such as for crack-freedeposition and lithography of thin films.

Photoresist is applied to the upper surface of the substrate to obtain aresist film, e.g., a film thickness of about 1 micron. The substrate isbaked at a temperature that will minimize or prevent warping of thesubstrate yet cure the resist. Generally, the substrate is soft-baked ata temperature of less than about 100° C., e.g., less than about 90° C.

The resist film is exposed, e.g., to UV light, through a mask totransfer a pattern of active regions onto the resist. The exposed resistis developed to remove undesired resist. A thin conductive layer, e.g.,a layer having a thickness of typically less than a micron, e.g., fromabout 0.1 to about 1 microns in thickness, is applied to the upperlayer. For a given surface area, thinner layers have a higher resistanceand will create higher temperatures in a microfluidic device. The resistis removed leaving behind the patterned active regions. Duringprocessing, the backside of the PCB may be protected using a resistfilm.

Alternatively, metal film can be deposited over the entire upper surfaceof the substrate. Resist is applied to the metal film and exposed andthe resist developed. Unwanted metal film is removed leaving thepatterned active regions.

After forming the pattern of active regions, the substrate is cleanedand a typically non-conductive barrier layer applied to protect theactive regions and prevent unwanted electrical contact to the activeregions. In general, the barrier is applied at temperatures of less thanabout 35° C.

Once the active regions have been patterned, contacts are applied toprovide electrical communication between the active regions and dataacquisition and control circuitry.

While the invention has been illustratively described herein withreference to certain aspects, features and embodiments, it will beappreciated that the utility and scope of the invention is not thuslimited and that the invention may readily embrace other and differingvariations, modifications and other embodiments. For example, the sametechniques for reducing the number of leads may be applied to othertypes of components, not just resistors. The invention therefore isintended to be broadly interpreted and construed, as comprehending allsuch variations, modifications and alternative embodiments, within thespirit and scope of the ensuing claims.

1. A system comprising: a microfluidic device comprising a channelconfigured to receive a fluidic sample; an electrical pathway comprisinga solid element disposed in thermal communication with the channel, thesolid element having an electrical characteristic dependent on thetemperature of the solid element; an electrical energy source inelectrical communication with the electrical pathway; an electricalmeasurement device in electrical communication with the solid elementand configured to obtain data indicative of the electricalcharacteristic of the solid element; and a computer-readable mediumcomprising executable software for operating the system, wherein thecomputer-readable medium comprises: code to provide a first actuationstate of the electrical energy source, wherein a first electricalcurrent, selected so as to raise the temperature of the fluidic sample,flows through the solid element; code to provide a second actuationstate of the electrical energy source, wherein a second, lowerelectrical current flows through the solid element; and code to receivedata indicative of the electrical characteristic of the solid elementfrom the electrical measurement device.
 2. The system of claim 1,wherein the computer-readable medium additionally comprises code todetermine a temperature of the solid element based on the dataindicative of the electrical characteristic of the solid elementreceived from the electrical measurement device.
 3. The system of claim2, wherein the data indicative of the electrical characteristic of thesolid element are obtained while the electrical energy source is in thesecond actuation state.
 4. The system of claim 1, wherein thecomputer-readable medium further comprises: code to determine atemperature of the solid element based on the data indicative of theelectrical characteristic of the solid element received from theelectrical measurement device, wherein the data is indicative of theelectrical characteristic of the solid element when the electricalenergy source is in the second actuation state; code to compare thetemperature of the solid element with a predetermined temperature value;and code to repeat providing the first and second actuation states ofthe electrical energy source if the temperature of the solid element isless than the predetermined temperature value.
 5. The system of claim 4,wherein the computer-readable medium further comprises code to vary, atleast once, at least one of the first and second electrical currentswhen repeating the providing of the first and second actuation states.6. The system of claim 1, wherein: the solid element has a thermaldissipation constant (DC); and the code to provide a first actuationstate of the electrical energy source is configured to cause the solidelement to dissipate a power k, wherein the ratio k/DC is greater thanor equal to 40° C. and less than 250° C.
 7. The system of claim 1,wherein the solid element is external to the channel.
 8. The system ofclaim 1, wherein the first actuation state has a duty cycle of no morethan about ninety percent and the second actuation state has a dutycycle of no more than about five percent.
 9. The system of claim 3,wherein the electrical characteristic of the solid element is atemperature-dependent resistance of the solid element.
 10. A systemcomprising: a microfluidic device comprising a channel configured toreceive a fluidic sample; an electrical pathway comprising a solidelement disposed in thermal communication with the channel, the solidelement having an electrical characteristic dependent on the temperatureof the solid element; an electrical energy source in electricalcommunication with the electrical pathway, the electrical energy sourcehaving a first actuation state and a second actuation state, the firstactuation state comprising a first current selected so as to raise thetemperature of the fluidic sample; and an electrical measurement devicein electrical communication with the solid element and configured toobtain data indicative of the electrical characteristic of the solidelement.
 11. The system of claim 10, wherein: during the first actuationstate, the solid element is configured to receive a first electricalpotential causing the first electrical current to flow through theelectrical pathway; and during the second actuation state, the solidelement is configured to receive a second electrical potential causing asecond, lower, electrical current to flow through the electricalpathway; and the data indicative of the electrical characteristic of thesolid element is indicative of the electrical potential required tocause the second, lower, electrical current to flow through theelectrical pathway.
 12. The system of claim 10, wherein: the solidelement has a thermal dissipation constant (DC); and during the firstactuation state of the electrical energy source, the solid element isconfigured to dissipate a power k, wherein the ratio k/DC is greaterthan or equal to 40° C. and less than 250° C.
 13. The system of claim10, wherein the data indicative of the electrical characteristic of thesolid element are obtained while the electrical energy source is in thesecond actuation state.
 14. The system of claim 13, wherein theelectrical characteristic of the solid element is atemperature-dependent resistance of the solid element.
 15. The system ofclaim 10, wherein the solid element is external to the channel.
 16. Thesystem of claim 11, wherein the first actuation state has a duty cycleof no more than about ninety percent and the second actuation state hasa duty cycle of no more than about five percent.
 17. A method forcontrolling a fluidic sample in a microfluidic device, the methodcomprising: introducing the fluidic sample into the microfluidic device,wherein the microfluidic device comprises: a channel configured toreceive the fluidic sample; an electrical pathway comprising a solidelement disposed in thermal communication with the channel, the solidelement having an electrical characteristic dependent on the temperatureof the solid element; an electrical energy source in electricalcommunication with the electrical pathway, the electrical energy sourcehaving a first actuation state and a second actuation state; and anelectrical measurement device in electrical communication with the solidelement and configured to obtain data indicative of the electricalcharacteristic of the solid element; causing the electrical energysource to operate in the first actuation state by applying a firstelectrical potential across the solid element, whereby a firstelectrical current, selected so as to raise the temperature of thefluidic sample, flows through the electrical pathway; causing theelectrical energy source to operate in the second actuation state byapplying a second electrical potential across the solid element, wherebya second, lower, electrical current flows through the electricalpathway; receiving data indicative of the electrical characteristic ofthe solid element from the electrical measurement device; anddetermining a temperature of the fluidic sample based upon the dataindicative of the electrical characteristic of the solid elementreceived from the electrical measurement device.
 18. The method of claim17, wherein: the solid element has a thermal dissipation constant (DC);and during the first actuation state of the electrical energy source,the solid element is configured to dissipate a power k, wherein theratio k/DC is greater than or equal to 40° C. and less than 250° C. 19.The method of claim 17, wherein the receiving data indicative of theelectrical characteristic of the solid element from the electricalmeasurement device occurs while the electrical energy source is in thesecond actuation state.
 20. The method of claim 19, wherein theelectrical characteristic of the solid element is atemperature-dependent resistance of the solid element.
 21. The method ofclaim 17, wherein the solid element is external to the channel.
 22. Themethod of claim 17, wherein the first actuation state has a duty cycleof no more than about ninety percent and the second actuation state hasa duty cycle of no more than about five percent.
 23. The method of claim17, wherein the data indicative of the electrical characteristic of thesolid element is indicative of the electrical potential required tocause the second, lower, electrical current to flow through theelectrical pathway.